Probe Liquids Research Articles

A statistical model for the wettability of surfaces with heterogeneous pore geometries

We describe a new approach to modeling the wetting behavior of micro- and nano-textured surfaces with varying degrees of geometrical heterogeneity. Surfaces are modeled as pore arrays with a Gaussian distribution of sidewall reentrant angles and a characteristic wall roughness. Unlike conventional wettability models, our model considers the fraction of a surface’s pores that are filled at any time, allowing us to capture more subtle dependences of a liquid’s apparent contact angle on its surface tension. The model has four fitting parameters and is calibrated for a particular surface by measuring the apparent contact angles between the surface and at least four probe liquids. We have calibrated the model for three heterogeneous nanoporous surfaces that we have fabricated: a hydrothermally grown zinc oxide, a film of polyvinylidene fluoride (PVDF) microspheres formed by spinodal decomposition, and a polytetrafluoroethylene (PTFE) film with pores defined by sacrificial polystyrene microspheres. These three surfaces show markedly different dependences of a liquid’s apparent contact angle on the liquid’s surface tension, and the results can be explained by considering geometric variability. The highly variable PTFE pores yield the most gradual variation of apparent contact angle with probe liquid surface tension. The PVDF microspheres are more regular in diameter and, although connected in an irregular manner, result in a much sharper transition from non-wetting to wetting behavior as surface tension reduces. We also demonstrate, by terminating porous zinc oxide with three alternative hydrophobic molecules, that a single geometrical model can capture a structure’s wetting behavior for multiple surface chemistries and liquids. Finally, we contrast our results with those from a highly regular, lithographically-produced structure which shows an extremely sharp dependence of wettability on surface tension. This new model could be valuable in designing and evaluating processes for manufacturing liquid-repellent surfaces on an industrial scale.

Perfluorinated ProDOT monomers for superhydrophobic/oleophobic surfaces elaboration

Here, we report for the first time the use of the Staudinger-Vilarrasa reaction to synthesize perfluorinated 3,4-propylenedioxythiophene (ProDOT) monomers for preparing hydrophobic/oleophobic surfaces. The efficiency of this reaction is shown with various perfluorinated chains. The prepared monomers are nicely polymerized to form homogeneous perfluorinated surfaces. The study of these surfaces reveal extremely high hydrophobic features with para and superhydrophobic properties (high and low water adhesion). Using different organic probe liquids (diiodomethane and hexadecane), it has been shown that the surfaces present also high oleophobic features. This work also shows a nice impact of the perfluorinated chain length on the surface morphologies.

Wettability of a Single Carbon Fiber.

Wettability as determined from contact angle measurements is a suitable parameter for characterizing the physical bonding of a polymer matrix and reinforcing fibers, but it is very challenging to measure the capillary force exerted by a probe liquid on a fiber accurately for very fine fibers such as single carbon fibers. Herein, we propose an innovative method for measuring dynamic contact angles with a tensiometer, considering both the intrinsic variability of the carbon fiber diameter and the extremely small amplitude of the capillary forces, allowing the measurement of reliable dynamic contact angles over a large range of contact line velocities. The analysis of the contact angle dynamics by the molecular-kinetic theory permits us to check the relevancy of the measured contact angles and to obtain the static contact angle value, improving the prospect of employing tensiometry to better understand the wetting behavior of carbon fibers.

Empirical Evidence for Roughness-Dependent Limit in Observation of Odd-Even Effect in Wetting Properties of Polar Liquids on n-Alkanethiolate Self-Assembled Monolayers.

Substrate roughness influences the wetting properties of self-assembled monolayers (SAMs), but details on this dependency at the sub-nanometer level are still lacking. This study investigates the effect of surface roughness on interfacial properties of n-alkanethiolate SAMs, specifically wetting, and confirms the predicted limit to the observation of the odd-even effect in hydrophobicity. This article studies static contact angles of polar and nonpolar probe liquids on a series of n-alkanethiolate SAMs on surfaces with tunable roughness. We prepared Ag surfaces with root-mean-square roughness (Rrms) of ∼0.6-2.2 nm and compared the wetting properties of n-alkanethiolate SAMs fabricated on these surfaces. We measured the static contact angles, θs, formed between SAM and probe liquids [water, glycerol, and hexadecane]. Hexadecane showed an odd-even effect on all surfaces irrespective of the degree of roughness. Polar liquids (water and glycerol), however, showed a dependency on the roughness of the substrate with an odd-even effect observable only on smooth, but not rougher (Rrms ≥ 1.15 nm), surfaces. These results confirm that the previously predicted limit to observation of the odd-even effect in hydrophobicity (here extended to polar liquids) is real. From the results with glycerol, we infer that this limit is not limited just to hydrophobicity but may extend to other polar liquids. Results from hexadecane, however, suggest that this limit may not be a universal property of the SAM.

Time‐dependent changes of surface properties of polyether ether ketone caused by air plasma treatment

AbstractThe effect of air plasma treatment on wetting and energy properties, surface composition and morphology of polyether ether ketone (PEEK) was investigated. The influence of the storage time on the surface properties of plasma‐treated polymer plate was also examined. The properties were determined by advancing and receding contact angle measurements, Fourier transform infrared spectroscopy supported by theoretical spectrum modelling, X‐ray photoelectron spectroscopy and optical profilometry. Three theoretical approaches were used in the determination of the apparent surface free energy of the untreated and plasma‐treated PEEK samples from the measured contact angles of probe liquids (water, formamide, diiodomethane): the contact angle hysteresis method, the Owens and Wendt approach and the Lifsthitz − van der Waals acid–base approach. It was found that air plasma treatment of PEEK causes significant chemical and morphological changes of the polymer surface, which are reflected in the decrease of contact angles from 83.4° to 11.7° for water after 180 s plasma treatment. This is due to the formation of polar functional groups resulting in the increase of the surface hydrophilicity. After plasma treatment the apolar component of the surface free energy practically does not change, while the polar component increases significantly, especially for plates treated for 180 s, from 0 to 19.6 mJ m−2. In addition, the modified PEEK surface is not stable during storage and it acquires more hydrophobic character. © 2016 Society of Chemical Industry

Surface characteristics of silver oxide thin film electrodes for supercapacitor applications

In this paper the preparation of nano-structured silver oxide thin films with different oxidation states with promise as electrode materials for supercapacitors by reactive magnetron sputtering is investigated. The chemical, structural, surface morphological dependence of the films on oxygen gas flow rate and deposition power were examined by scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX). The average thickness of the films was controlled in the range of ≈50–330nm. The XRD spectra of the films indicated the formation of bi-phase films comprised of silver and silver oxides with different oxidation states. The wettability of the films in contact with different probing liquids was investigated by measuring the contact angles using a goniometer. It was shown that the silver oxide films are relatively hydrophilic and increasing oxygen flow rate increases the wettability of the films toward water as a result of an increase in the oxidation state of the films and consequently clustering of electrons in polar molecules of water around the oxides at higher oxidation states. This is further confirmed by the analysis of the surface energy measurements.

Contact Angles and Wettability of Cellulosic Surfaces: A Review of Proposed Mechanisms and Test Strategies

Contact angle methods are widely used to evaluate the wettability of cellulose-based surfaces and to judge their suitability for different applications. Wettability affects ink receptivity, coating, absorbency, adhesion, and frictional properties. There has been a continuing quest on the part of researchers to quantify the thermodynamic work of adhesion between cellulosic surfaces and various probe liquids and to account for such components of force as the London/van der Waals dispersion force, hydrogen bonding, and acid and base interactions. However, due in part to the rough, porous, and water-swellable nature of cellulosic materials, poor fits between various theories and contact angle data have been observed. Such problems are compounded by inherent weaknesses and challenges of the theoretical approaches that have been employed up to this point. It appears that insufficient consideration has been given to the challenging nature of cellulosic materials from the perspective of attempting to gain accurate information about different contributions to surface free energy. Strong hysteresis effects, with large differences between advancing and receding contact angles, have been overlooked by many researchers in attempting to quantify the work of adhesion. Experimental and conceptual approaches are suggested as potential ways to achieve more reliable and useful results in future wettability studies of cellulosic surfaces.

Immersion Calorimetric Study of UV/ozone Processed Nano-PS Fibers

Nano- polystyrene (PS) fibers have potential applications in tissue engineering and regenerative medicine owning to their highly selective sorption capacity and good biocompatibility. They are not necessarily appropriate substrates for protein adsorption and cell attachment due to their hydrophobic nature. Surface treatments are often required before they can be successfully used in these applications. This paper reports on the surface processing of nano-PS fibers by using a perfusion UVozone reactor. The effect on surface characteristics and immersion enthalpy were investigated. The UV-ozone surface processing introduced oxygen-containing functional groups to the PS fibers, which improved the surface wettability significantly. Chain scission and cross-linking process also occur during the UV-ozone processing and lead to a change in microstructural topography of PS fiber surface microstructural topography. The study, using water and heptane as a polar probe and nonpolar probe liquid respectively, revealed that the enthalpy varies with oxygen concentration and is probe liquid dependant. The water enthalpy increases with surface oxygen concentration, but it is insensitive to the non-polar probe liquid heptane. It is believed that the presence of low molecular weight oxidised molecules (LMWOM) and surface roughing of PS fibers, as a result of UV-ozone processing, contributed to the water enthalpy increase after the surface oxygen reaches saturation level. This study shows that immersion calorimetry is a useful technique which simplifies considerably the analysis of porosity and chemical nature of polystyrene nanofibers.

Investigation of moisture damage resistance of GTR-modified asphalt binder by static contact angle measurements

More than 300 million scrap tires are being generated every year in the United States and their proper disposal has been a major concern from environmental safety and economical point of views. Using these scrap tires as ground tire rubber (GTR) in asphalt mixes is becoming a common practice now a days. A number of research studies have been conducted to characterize the mechanistic properties of GTR modified asphalt. However, a very limited research has been conducted to evaluate their moisture susceptibility, particularly through fundamental science approaches. Moisture damage or stripping, the loss of strength and stiffness of asphalt mixes due to the existence of moisture, of asphalt pavements is one of the major concerns to the state and federal highway agencies for many years. The objective of this research was to evaluate moisture susceptibility of GTR modified asphalt binders through a surface science approach, in which surface free energy (SFF) components were estimated by following the sessile drop method. In the experimental plan, a commonly used performance grade (PG) binder (PG 64-22) modified with different dosages (0%, 10%, 15%, and 20%) of GTR were tested for static contact angles. Three different probe liquids, namely: water, formamide and ethylene glycol were used to determine contact angle with binder samples using an optical contact angle analyzer (OCA) device. The SFE of five different aggregates, namely: sandstone, gravel, granite, basalt and limestone, were used to evaluate the work of adhesion and compatibility ratio (CR). The SFE results of unmodified and modified binders showed that SFE increased due to the addition of GTR in binder. The work of adhesion of a binder aggregate system also increased with an increase of the GTR content when measured in the absence of water, but it decreased gradually in the presence of water. Granite and basalt showed lowest work of adhesion, whereas sandstone showed the highest work of adhesion. The CR data showed that granite and basalt were the most compatible aggregates with GTR modified binders. The CR value also had an increasing trend with an addition of higher GTR content, indicating an improved moisture resistance. The CR values of all asphalt aggregate systems were found to be greater than 1.5 with 20% GTR, indicating “good” moisture susceptibility.

Critical Surface Tension of HSF54 Carbon Composite Electrodes on Paper Substrate

The wettability of the carbon composite electrode on paper substrate was studied using contact angle measurement. Y-Shield HSF54 carbon composite material were coated on paper substrates with a mask using squeegee coating technique. Contact angles of the ten polar probe liquids with the HSF54 electrodes were measured using a customed-built image acquisition system. With the contact angles obtained for each probe liquid placed on the electrodes, the surface tension of the electrodes was determined using Fox-Zisman theory. The result showed that the critical surface tension of HSF54 carbon composite electrodes was 37.2 mN/m which is considered to be moderately hydrophobic.

Sensing of liquid droplets with a scanning near-field microwave microscope

We demonstrate a scanning near-field microwave microscope based on an interferometric technique for dielectric characterization in liquid media. The instrumentation combines a vector network analyzer, an interferometry-based matching network and an evanescent microwave coaxial probe. The measurement sensitivity of the measured microwave signals can be enhanced at the desired frequency in the range 1–26GHz. Other advantages such as simplicity of operation and high spatial resolution are also achieved. As a demonstration, we have observed a high electromagnetic coupling between the probe tip and liquid droplets featuring emergent applications in biological and chemical researches.

Fabrication of Superhydrophilic Polyester Fabrics Using Various TiO2Colloidal Dispersions

The purpose of this research was to apply four stable colloidal dispersions, combining di erent TiO2 coreshell nanoparticles, onto polyester fabric, according to exhaustion procedure, in order to enhance fabrics' water and moisture absorption properties. Firstly, all colloidal dispersions were analysed using transmission electron microscopy (TEM) and dynamic light scattering (DLS) and secondly, the morphologies of TiO2 modi ed PES surfaces were evaluated by scanning electron microscopy (SEM). In addition, the superhydrophilic character of brous surfaces was investigated by water contact angle measurement. For this purpose, dynamic tensiometry was employed by measuring the changes in the net force on a surface during the repeated immersion and emersion of a solid into a probe liquid. The obtained results show excellent wettability of TiO2 upgraded PES fabrics depending on the type of applied nano-sized TiO2-hybride.

Effect of surface modification on starch biopolymer wettability

The main goal of this paper was to determine wettability properties of native starch and starch modified by antibacterial solution of chitosan or lysozyme as a function of temperature and/or after plasma treatment. Contact angles for polar water (and formamide) and nonpolar diiodomethane on glass plates covered with starch film before and after gelatinization (25 °C, 37 °C and 60 °C) were measured. The surface free energy of solid (gel) was determined from the advancing and receding contact angles of these probe liquids, using the contact angle hysteresis model (CAH). The plasma treatment was immediately followed by a sharp decrease of contact angle. In the course of ageing the contact angle also decreased due to the reorientation of polar groups into the surface layer of polymer. Also a sharp decrease in negative values of WS takes place which indicates more hydrophilic character of starch after plasma treatment. The same was after lysozyme modification but to a smaller extent. The opposite trend of changes was visible after chitosan solution treatment. The surface characterization was observed by FTIR measurements and surface topography from the profilometer images and such measurements well support the contact angle results. The arrangement of temperature, plasma treatment or biological modification can lead to improvement of starch surface wettability and stability. The obtained results open up possibilities of starch application in food, paper and packaging industries as well as in pharmacy.

Effect of surface modification on starch/phospholipid wettability

Abstract The objective of this work was to determine surface properties of biopolymer film as a function of temperature and/or after plasma treatment with the aim of further wettability tests and biocompatibility. The examined biopolymers were native starch and starch modified by solution of PC (phosphatidylcholine). Contact angles for polar water (and formamide) and nonpolar diiodomethane on the glass plates covered with starch film before and after gelatinization (25 °C, 37 °C and 60 °C) were measured for the base and plasma modified film. The surface free energy of solid (gel) was determined from the advancing and receding contact angles of these probe liquids, using the contact angle hysteresis model (CAH). The plasma treatment was immediately followed by a sharp decrease of contact angle and in the course of ageing the contact angle also decreased due to the reorientation of polar groups into the polymer surface layer. To depict better changes in hydrophilic/hydrophobic character of the starch surface before and after modification the work of water spreading, W S was calculated. A sharp decrease in negative values of W S takes place which indicates more hydrophilic character of starch surface after modification. Surface morphology and roughness were studied by optical profilometer and FTIR measurements which confirm the contact angle results. It was also proved that a combination of input treatment parameters led to improvement of biopolymer wettability and provided possibilities of starch application where uniform thin films are required.

Effect of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and phospholipase A2 (PLA2) on surface properties of silica materials

Abstract The effect of phospholipid DPPC monolayer on surface properties of powdered quartz (SiO 2 /DPPC ML ) and silicon wafer (Si/DPPC ML ) before and after phospholipase A 2 (PLA 2 ) action was determined. DPPC on quartz particles was deposited from chloroform solution and the monolayer on silicon surface by using the Langmuir–Blodgett technique. The surface free energy and its components of powdered quartz before and after contact with PLA 2 in NaCl solution were determined by imbibition–drainage method (I–DM), which is based on the capillary rise method. The probe liquids (water, formamide and diiodomethane) wick the powder bed. The DPPC monolayer supported on silicon after the contact with NaCl or Tris solution (without and with enzyme PLA 2 ) was monitored by contact angle measurements of the probe liquids. In order to visualize the changes in the DPPC monolayer structure, its topography images were obtained from AFM. The obtained results show that most evident changes in wettability, i.e. the apparent surface free energy of DPPC monolayer supported on silicon (Si/DPPC ML ) and treated with PLA 2 occur within the first 15 min after initiation of the hydrolysis. The analysis of the topography images of the phospholipid monolayer clearly reveals that degradation of monolayer occurs. The modification of quartz powder with DPPC resulted in decreased apparent surface free energy, which however increased after the PLA 2 action, up to the value obtained for bare quartz surface. This again revealed the hydrolysis reaction. The influence of PLA 2 on the surface free energy changes of DPPC monolayer deposited on silicon correlated with those observed for quartz powder. These results pointed out a complete hydrolysis of DPPC by PLA 2 enzyme. The AFM images shed more light on the relation between DPPC monolayer topography and the apparent surface free energy changes taking place during the hydrolysis catalyzed by PLA 2 .

Surface Properties of Aerated Ion-induced Whey Protein Gels.

Aerated whey protein gels were formed using calcium chloride, magnesium chloride or iron (II) chloride induced gelation of pre-denatured protein dispersions. The structure of the obtained gel surface depends on the type and concentration of added salt. Higher cation concentration produced gels a with higher quadratic mean of the surface roughness and maximum roughness height. Aerated gels of optimal properties for retaining air bubbles were characterized by similar surface roughness. The surface topography is mainly responsible for changes in the wettability. The contact angle of the probe liquid sample depends on the liquid surface tension components. An approach based on the contact angle hysteresis (CAH) is suitable for determining the total value of the apparent surface free energy of such materials. An approach based on the components of apparent surface free energy (LWAB) only allows the calculation of the dispersion component and electron donor parameter of energy in the case of added magnesium and iron salt. Wettability, depending on the nature of the surface, can be described for the hydrophilic surface by the Wenzel model, and for the hydrophobic surface by the Cassie – Baxter model.

In Vitro Inhibitory Effect of Licoricidin on Human Cytochrome P450s

Licoricidin isolated from Glycyrrhiza uralensis is known to have anticancer, anti-nephritic, anti-Helicobacter pylori, and antibacterial effects. In this study, a cocktail probe assay and liquid chromatography-tandem mass spectrometry (LC-MS/ MS) were used to investigate the modulating effect of licoricidin on cytochrome P450 (CYP) enzymes in human liver microsomes. When licoricidin was incubated at 0-25 μM with CYP probes for 60 min at 37oC, it showed potent inhibitory effects on CYP2B6-catalyzed bupropion hydroxylation and CYP2C9-catalyzed diclofenac 4’-hydroxylation with half maximal inhibitory concentration (IC50) values of 3.4 and 4.0 μM, respectively. The inhibition mode of licoricidin was revealed as competitive, dose-dependent, and non-time-dependent, and following the pattern of Lineweaver-Burk plots. The inhibitory effect of licoricidin has been confirmed in human recombinant cDNA-expressed CYP2B6 and 2C9 with IC50 values of 4.5 and 0.73 μM, respectively. In conclusion, this study has shown the potent inhibitory effect of licoricidin on CYP2B6 and CYP2C9 activity could be important for predicting potential herb-drug interactions with substrates that mainly undergo CYP2B- and CYP2C9- mediated metabolism.

A method for estimating interfacial tension of liquid crystal embedded in polymer matrix forming PDLC

ABSTRACTA simple and convenient method based on sessile drop technique for measuring surface tensions of polymer and nematic liquid crystal (LC) is described. Contact angles formed by drops of probe liquids and a nematic LC on a photocurable polymer were measured. The surface energies were evaluated using the Fowkes method, Neumann's equation, and new equations developed based on Neumann's approach. The values of surface tensions were used to evaluate the interfacial interaction in term of work of adhesion between the LC and polymer. Further, the effect of dichroic dye on the extent of interaction and work of adhesion was examined by measuring contact angle in consequence of dye addition. A difference in work of adhesion between the lower and higher dye‐doped LC droplets gave an indication of affinity relationship between polymer and LC molecules. A change in work of adhesion resulted in variability of nematic director configurations inside phase separated LC droplets embedded in polymer matrix; when viewed under polarizing optical microscope. Thus, our approach of estimating surface energy of polymer and LC has found to be useful in determining interaction at polymer–LC interface. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41137.

Surface Gibbs energy interaction of phospholipid/cholesterol monolayers deposited on mica with probe liquids

The mica supported binary monolayers containing phospholipids: 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC), 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DPPG), and cholesterol (Chol), mixed at different molar fractions, were investigated by measurements of the contact angles of water, formamide and diiodomethane. This allowed calculation of apparent surface Gibbs energy (further in the paper termed as ‘surface free energy’) of the monolayers according to the theoretical approach developed by Chibowski (contact angle hysteresis model, CAH). Then, based on the surface free energy values, the molar interaction Gibbs energy of the lipid molecules with the given probe liquid was evaluated. These values correlate with the values of excess area, interpreted as an indicator of the condensing effect of cholesterol on phospholipid monolayers at the air–water interface. The results indicate that the thermodynamic parameters of interactions depend on the monolayer composition and the probe liquid used to their determination. Changes of the parameters are discussed in relation to the monolayer packing, ordering, tilting of the molecules, and properties of the probe liquids as well.

Automated Drop-on-Fiber contact angle measurement using a microrobotic platform

Abstract Measuring contact angles on single fibers enables the separation of structural factors from surface chemistry factors. Current Drop-on-Fiber (DOF) contact angle measurement methods for natural fibers generally, and for pulp/paper fibers specifically, present a number of challenges. These are manipulation and mounting of the microscale fibers, straightening natural fibers to produce repeatable axisymmetric droplet conformation, measuring the droplet volume excluding the fiber volume and also human errors due to manually performed tasks. This paper presents a novel method to measure contact angles in DOF systems and overcome the above mentioned challenges using microrobots. The proposed microrobotic platform is capable of handling natural and synthetic fibrous materials in microscale, and dispensing probe liquid droplets down to 12 nl. It measures contact angle values using computer vision and a droplet-profiledetection algorithm. It reports the contact angle values as a function of volume of the droplet. The paper validates the capabilities of the proposed platform by applying three commonly used probe liquids: deionized water, ethylene glycol and diiodomethane for measuring contact angles on glass and pulp fibers. Finally, the results are compared with a picoliter contact angle measurement approach.